[asterisk-users] OT - Incoming fax cuts ADSL line

[Cary Fitch]

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com
[mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of James Sharp
Sent: Wednesday, May 16, 2012 11:19 AM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [asterisk-users] OT - Incoming fax cuts ADSL line

On 5/16/2012 12:07 PM, Tim Nelson wrote:
> ----- Original Message -----
>> Hi,
>>
>> I'm facing a strange situation.
>> Though it's not directly related to Asterisk, I do think it is
>> interesting to this mailing list.
>>
>>
>> The setup is a single line which is split between an ADSL
>> modem/routeur and a fax machine (Asterisk was removed from the
>> equation).
>>
>> Any time the fax machine rings (incoming fax), the ADSL service is
>> troubled to the VPN users are disconnected.
>> It can be reproduced at will.
>>
>> I've changed the ADSL filter twice (a different unit, then a
>> different
>> model) without any visible change.
>> What could explain this ?
>>
>
> I've experienced this quite a few times, and after working with a local
telco, it has become policy to not place ADSL on lines where fax is going to
be used. I'm unsure of the exact technical reasons behind this other than
'the fax signals/frequencies interfere with the ADSL signalling/frequencies
used on the circuit'. It sounds like you might want to separate your
fax/ADSL lines.
>
> --Tim

You might also be able to limit the Fax machines maximum transmission 
rate so the modem's transmission spectrum doesn't inch up into where the 
ADSL service is.


ADSL is transmitted at a relatively low frequency using phase modulated
carriers to achieve the bandwidth. It could be about 32 different
phase/level locations on 360 degree/level "pie chart" or vector scope.

The actual frequencies of the carrier are moderately low, maybe 100 to 200
kcps.

Voice is low density.  Faxes and modems are high density and "loud".  They
can splatter or have harmonics that can confuse the local DSL demodulator.

As others have said, the best thing to try is the best filters you can get
between the phone line and the DSL demod, and maybe two filters in series.
If that doesn't work, put the fax on a different line than the DSL, which
could cost you money.  Paying for better filters or two of them is less
expensive than separate lines.

Or move the DSL to an alternate existing voice only line, since you probably
don't want to change the fax number

Contents of this message were dredged from foggy memory.

Cary Fitch